This application is based on Japanese Patent Application No. 2002-273238 filed on Sep. 19, 2002, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a solid-state image-sensing device of which all the pixels can perform image sensing operation with identical timing, and more particularly to a solid-state image-sensing device of which each pixel is provided with an integrating circuit.
2. Description of the Prior Art
Conventional solid-state image-sensing devices are classified roughly into a CCD type and a MOS type, according to the type of the means they use to read out the photoelectric charges generated in photoelectric conversion elements. CCD-type solid-state image-sensing devices transfer the photoelectric charges while accumulating them in potential wells, and have the disadvantage of narrow dynamic ranges. On the other hand, MOS-type solid-state image-sensing devices read out the electric charges accumulated in the pn-junction capacitance of photodiodes through MOS transistors.
In a MOS-type solid-state image-sensing device, while vertical scanning and horizontal scanning are performed, image sensing operation is performed in one pixel after another, and the image signals output from one pixel after another are serially fed out of the solid-state image-sensing device. Thus, each pixel performs image sensing operation with different timing. As a result, when an image of a subject in a constantly changing state is taken, it is impossible to perform image sensing operation under identical conditions in all the pixels. This may eventually lead to distortion in the obtained image.
To overcome this inconvenience, Japanese Patent Application Laid-Open No. 2002-77733 proposes a solid-state image-sensing device of which each pixel is provided with two integrating circuits so that all the pixels perform image sensing operation with identical timing. The configuration of each pixel of this conventional solid-state image-sensing device is shown in FIG. 10. The pixel shown in FIG. 10 includes a photoelectric conversion circuit 100 that produces an electric signal commensurate with incident light, a capacitor C1 that integrates the electric signal fed from the photoelectric conversion circuit 100, a capacitor C2 that samples the electric signal integrated by the capacitor C1, a MOS transistor T4 that current-amplifies the electric signal sampled by the capacitor C2, a MOS transistor T5 that electrically connects and disconnects the capacitors C1 and C2 to and from each other, a MOS transistor T6 that functions as a switch for resetting the capacitor C2, and a MOS transistor T3 that functions as a switch for outputting, as an image signal, the electric signal fed from the MOS transistor T4.
In the solid-state image-sensing device having each pixel thereof configured as shown in FIG. 10, the photoelectric conversion circuit 100 and the MOS transistor T5 provided in all the pixels operate with identical timing. Thus, in all the pixels, the electric signal obtained through image sensing operation performed with identical timing is integrated by the capacitor C1, and is then sampled by the capacitor C2. Then, the MOS transistor T5 in each pixel is turned off, and thereafter horizontal scanning and vertical scanning are performed so that, from one pixel after another, an amplified image signal commensurate with the electric signal sampled by the capacitor C2 is output.
However, in the above-described configuration, where each pixel includes two integrating circuits connected successively with a switch between them, when the MOS transistor T5 is turned on, the capacitors C1 and C2 become connected in parallel. Thus, the signal electric charge accumulated in the first-stage capacitor C1 is divided between the capacitance of this capacitor C1 and the capacitance of the second-stage capacitor C2. Specifically, assuming that the capacitors C1 and C2 have equal capacitances in FIG. 10, when first the MOS transistor T5 is turned on to make the capacitor C2 perform sampling and then the MOS transistor T5 is turned off, half the electric charge integrated by the capacitor C1 remains in the capacitor C2. This reduces the sensitivity of the pixel in half compared with that obtained when the level integrated by the capacitor C1 is fed out directly.
An object of the present invention is to provide a solid-state image-sensing device of which all the pixels can perform image sensing operation with identical timing and that outputs image signals with reduced loss.
To achieve the above object, according to one aspect of the present invention, a solid-state image-sensing device is provided with: a photoelectric conversion circuit that generates the amount of electric charge commensurate with the amount of incident light; a first integrating circuit that outputs a voltage obtained by accumulating the electric charge fed from the photoelectric conversion circuit; and a second integrating circuit that performs amplification based on the voltage fed from the first integrating circuit, performs integration by accumulating the electric charge obtained through the amplification, and outputs as an image signal the voltage obtained through the integration.
According to another aspect of the present invention, in a solid-state image-sensing device including a plurality of pixels, the pixels are each provided with: a photoelectric conversion circuit that generates the amount of electric charge commensurate with the amount of incident light; a first capacitor that accumulates the electric charge fed from the photoelectric conversion circuit; a switch having one end thereof connected to the node between the first capacitor and the photoelectric conversion circuit; a transistor having a first electrode, a second electrode, and a control electrode, and having the control electrode thereof connected to the other end of the switch; and a second capacitor having one end thereof connected to the second electrode of the transistor. Here, in all the pixels, the first capacitor performs integration simultaneously, and then the switch is turned on simultaneously so that the second capacitor performs integration simultaneously.
According to still another aspect of the present invention, a method for outputting an image signal in a solid-state image-sensing device includes: a step of generating the amount of electric charge commensurate with the amount of incident light; a step of outputting a first voltage obtained by accumulating the electric charge generated; and a step of outputting as an image signal a second voltage obtained by accumulating the electric charge obtained by amplifying the first voltage.